The present invention is directed to inkjet inks, and, more particularly, to inkjet inks that evidence improved directionality during jetting through nozzles of an inkjet cartridge.
Thermal inkjet print cartridges operate by rapidly heating a small volume of ink to cause the ink to vaporize and be ejected through one of a plurality of orifices so as to print a dot of ink on a recording medium, such as a sheet of paper. Typically, the orifices are arranged in one or more linear arrays in a nozzle member. The properly sequenced ejection of ink from each orifice causes characters or other images to be printed upon the paper as the printhead is moved relative to the paper. The paper is typically shifted each time the printhead has moved across the paper. The thermal inkjet printer is fast and quiet, as only the ink strikes the paper. These printers produce high quality printing and can be made both compact and affordable.
In one prior art design, the inkjet printhead generally includes: (1) ink channels to supply ink from an ink reservoir to each vaporization chamber proximate to an orifice; (2) a metal orifice plate or nozzle member in which the orifices are formed in the required pattern; and (3) a silicon substrate containing a series of thin film resistors, one resistor per vaporization chamber.
To print a single dot of ink, an electrical current from an external power supply is passed through a selected thin film resistor. The resistor is then heated, in turn superheating a thin layer of the adjacent ink within a vaporization chamber, causing explosive vaporization, and, consequently, causing a droplet of ink to be ejected through an associated orifice onto the paper.
A recurring problem over the years involves drop trajectory as the drop is ejected through the orifice onto a print medium, e.g., paper. For example, whenever an ink drop is ejected from an orifice of an orifice plate, a trailing portion or xe2x80x9ctailxe2x80x9d of ink moves with the drop. A small amount of the ink tail may separate and land on the outer surface of the plate as an ink droplet. Residual ink that collects on the orifice plate outer surface near the edges of the orifices may contact subsequently ejected ink drops, thereby altering the trajectory of those drops, which reduces the quality of the printed image.
Also, in the event that a substantial amount of residual ink accumulates on the orifice plate outer surface, a continuous liquid path between the ink within the orifice and the ink on the outer surface may be formed, thereby facilitating leakage of the ink out of the orifice. Further, when a substantial amount of ink accumulates on an orifice plate, this large pool of ink can interfere with drop ejection to the extent that no drops are ejected, i.e., a single drop is unable to pass through the large pool of ink. Moreover, the residual ink on the outer surface of the plate tends to trap minute particles, such as paper fibers, thereby interfering with the trajectory of subsequently-ejected drops.
For many years, ink jet technologies which have been developed to produce printheads for ink jet printers and the like have included sub-categories or sub-technologies directed specifically to forming the output ink ejection orifice plate or nozzle plate for controlling the ink drop patterns and ink trajectories onto an adjacent print medium. As is well-known to those skilled in the art, these orifice plate technologies include those for making silicon orifice plates, glass orifice plates, plastic orifice plates, and metal orifice plates of many different kinds of materials in each of the latter four types of orifice plate categories. In addition, these metal (e.g., nickel) orifice plate technologies include electroforming and electroplating processes including the fabrication of mandrels for making small geometry precision architecture orifice plates for attachment to thin film printhead substrates.
A variety of solutions have been patented that deal with drop trajectory. Many of these solutions comprise various alterations of the mechanical aspect of the printer, and examples of such solutions include: (1) off-setting the orifice from the resistor (see, e.g., U.S. Pat. No. 4,794,411, entitled xe2x80x9cThermal Ink-Jet Head Structure with Orifice Offset from Resistorxe2x80x9d, issued Dec. 27, 1988, to Howard H. Taub et al); (2) providing a drop detector for measuring flight characteristics of the drop and correcting the drop fire timing and image data to produce a higher quality image (see, e.g., U.S. Pat. No. 5,109,239, entitled xe2x80x9cInter Pen Offset Determination and Compensation in Multi-Pen Ink Jet Printing Systemsxe2x80x9d, issued Apr. 28, 1992, to Keith E. Cobbs et al); (3) eliminating the orifice plate (see, e.g., U.S. Pat. No. 5,371,527, entitled xe2x80x9cOrificeless Printhead for an Ink Jet Printerxe2x80x9d, issued Dec. 6, 1994, to Robert J. Miller et al); (4) reconfiguring the fabrication of a printhead to prevent bending of a nozzle member, which skews the nozzles, by forming the nozzles at a slight inward angle (see, e.g., U.S. Pat. No. 5,467,115, entitled xe2x80x9cInkjet Printhead Formed to Eliminate Ink Trajectory Errorsxe2x80x9d, issued Nov. 14, 1995, to Winthrop D. Childers); and (5) altering the architecture of the pen itself, that is, the structural portions, including passageways and peninsulas, that guide the ink to the firing chambers (U.S. Pat. No. 5,685,074, entitled xe2x80x9cMethod of Forming an Inkjet Printhead with Trench and Backward Peninsulasxe2x80x9d, issued Nov. 11, 1997, to Yichuan Pan et al).
Other solutions include: (1) providing selected portions of the orifice plate with wetting and non-wetting surface characteristics (see, e.g., U.S. Pat. No. 5,434,606, entitled xe2x80x9cOrifice Plate for an Ink-Jet Penxe2x80x9d, issued Jul. 18, 1995, to Suraj L. Hindagolla et al); and (2) treatment of the inner and outer surfaces of the orifice plate with self-assembled monolayers (see, e.g., U.S. Pat. No. 5,598,193, entitled xe2x80x9cTreatment of an Orifice Plate with Self-Assembled Monolayersxe2x80x9d, issued Jan. 28, 1997, to David J. Halko et al).
The ink itself has been reformulated in an attempt to overcome drop trajectory problems; see, e.g., (1) U.S. Pat. No. 5,098,476, entitled xe2x80x9cAdditive to Aqueous-Based Inks to Improve Print Qualityxe2x80x9d, issued Mar. 24, 1992, to Jeffrey P. Baker and (2) U.S. Pat. No. 5,112,399, entitled xe2x80x9cPlain Paper Inksxe2x80x9d, issued May 12, 1992, to Leonard Slevin et al.
In U.S. Pat. No. 5,098,476, a low molecular weight alcohol or a surfactant/defoming agent is added to reduce the surface tension of the ink and increase the surface wettability on paper. In U.S. Pat. No. 5,112,399, a viscosity modifier, such as an alginate, is used to increase the viscosity of the ink and thereby reduce spray and improve drop directionality.
Other modifications of inkjet inks involving surface tension control have also been undertaken, for a variety of reasons. For example, U.S. Pat. No. 5,880,758, entitled xe2x80x9cPrinter with Pen Containing a Low Dot Spread Black Ink and a High Dot Spread Color Inkxe2x80x9d, issued Mar. 9, 1999, to John L. Stoffel et al, discloses use of a surface tension in the range of 25 to 40 dyne/cm and a viscosity in the range of 1.5 to 10 cp for a relatively high dot spread ink (color ink) and a surface tension in the range of 45 to 65 dyne/cm and a viscosity in the same range as the color ink for a relatively low dot spread ink (black ink).
The combination of adjusting the surface tension of the ink and the contact angle of the ink and a solid surface, such as the print medium or a surface within the pen has also been considered; see, e.g., U.S. Pat. No. 5,626,655, entitled xe2x80x9cUse of Co-Surfactants to Adjust Properties of Inksxe2x80x9d, issued May 6, 1997, to Norman E. Pawlowski et al.
See also U.S. Pat. Nos. 4,555,062 and 4,583,690, both entitled xe2x80x9cAnti-Wetting in Fluid Nozzlesxe2x80x9d, issued Nov. 26, 1985 and Apr. 22, 1986, respectively, to Young S. You, which disclose a novel ionic surface preparation for nozzles used in spraying fluid droplets such as used in inkjet printers
In most cases, it appears that the prior art is primarily directed to adjusting the surface tension of the ink in order to deal with print quality issues, such as feathering of the print on the print medium, blooming, and the like.
Recent advances in inkjet printheads have moved in the direction of printing smaller and smaller drop volumes, with a reduction in diameter of the orifices in the printhead. As a consequence, the proper trajectory of the ejected ink droplet becomes more and more critical, and the ink must be carefully conditioned by its formulation to achieve the required trajectory. While the above-discussed patents are certainly suitable for their intended purposes, they do not deal with the issue of reduced drop volumes and their effects on droplet trajectories.
Thus, there remains a need for improved directionality of the ink as it leaves the printhead. Specifically, a need exists for an ink having reduced tail breakup and improved drop trajectory, with a concomitant improvement in print quality.
In accordance with the present invention, an inkjet ink for printing onto a print medium is provided. The inkjet ink evidences minimal tail breakup and improved drop trajectory, thereby evidencing improved print quality. The minimal tail breakup and improved drop trajectory are achieved by formulating the ink to include at least one surface active additive in an amount sufficient to provide the inkjet ink with a surface tension of at least 35 dyne/cm and a contact angle with the orifice plate in the range of 35 to 65 degrees.
Further in accordance with the invention, a method is provided for reducing tail breakup and improving drop trajectory in an inkjet ink, wherein at least one surface active additive is added to the ink.